Quenching

Steel is heated to a temperature higher than its transformation point to form austenite; then the steel is quickly cooled to form martensite. This process increases the hardness and strength of the steel without changing its shape, but the steel becomes more brittle.

Tempering

Heating the steel again after quenching adds elasticity and toughness, transforming the steel to a structure with high resistance to wear. However, the hardness is slightly reduced when compared with the state after quenching.

Annealing

This process is used to improve the structure and soften the steel since semi-hard steel, hard steel and alloy steel are difficult to cut due to their hardness.

Normalizing

This process is used to make the structure finer, uniformize carbides and remove residual stresses, thus making the steel easier to cut. Since hardenability also improves, normalizing is sometimes used as a pre-process before quenching.

Poor quenching or tempering may result in problems such as insufficient hardness, cracks, distortion and deterioration of dies, tools and jigs. In processed products such as mechanical parts, machining tools and functional parts, failures such as breaking or bending may occur after the products are built into machines, which may lead to user complaints and claims for damages.

Quenching and tempering under the right heat treatment conditions for the specific type of steel is essential to ensure the quality required for the application.

Click the button on the right for information on heat treatment for typical die steels and machinery structural steels.

Since metal is heated to high temperatures in heat treatment, the metal combines with moisture and oxygen in the air to produce oxides, which is called oxidation. Removing these oxides by grinding or other methods is necessary in the post-process. There may also be decarburization, which can occur when steel is heated. The carbon in the surface layer combines with oxygen in the air, resulting in carbon reduction.

Thermal’s quenching furnaces (A series) can reduce such oxidation and decarburization by providing a nitrogen atmosphere in the heating chamber. Methanol or propane gas may be added to the atmosphere depending on the target work.

For products such as mechanical parts, machining tools and functional parts, surface treatment may be applied to increase the hardness of only the surface layer instead of achieving uniform hardness over the entire work.

Carburizing and quenching

Nitrogen, methanol or propane gas is fed into the furnace. The gas is decomposed to cause carbon to permeate into the steel surface, creating a high-carbon steel layer only on the surface. Applying quenching and tempering to this state makes the surface harder and the deep portion moderately hard, producing a tough product.
This is a relatively deep surface hardening technique, which is widely used for automobile parts.

Nitriding

Ammonia or another gas is fed into the furnace. The gas is decomposed to cause nitrogen to permeate into the steel surface, creating a hard nitrided layer.
Nitriding methods including hard nitriding, gas soft nitriding and plasma nitriding. Nitriding is a relatively shallow surface hardening technique, which produces harder steel than that achieved by carburizing.
The process is widely used for parts such as bearings, gears and cylinder shafts.

There are various types of steel; their uses and heat treatment methods vary from one another.
The following tables 1 and 2 show the heat treatment characteristics of typical die steels and machinery structural steels specified in JIS (Japanese Industrial Standards).

Typical die steels

Steel type

Effective thickness（mm）

Die steel properties

Main applications

Water cooling

Oil cooling

Air cooling

Wear resistance

Heat resistance

Toughness

Deformation by heat treatment

Machinability

SK3

20

10

-

2

1

2

1

9

Simple dies for small lots

SKS3

-

70

5

4

2 to 3

4 to 5

4 to 5

8

General dies

SKD11

-

510

140

7 to 8

6

3 to 4

8 to 9

3

Precision heavy-load dies

SKD1

-

160

20

8 to 9

5

2

7 to 8

2 to 3

Drawing dies

SKD61

-

360

100

3 to 4

6

9

8

7 to 8

Shrink rings, hot dies

SKH9

-

60

10

7

7 to 8

4 to 5

4

6

Hot forged punches

Note 1)

The effective thickness is that where core hardness of HRC60 is obtained after quenching.

Note 2)

Die steel property rating 1 is the worst and 9 the best.

Typical machinery structural steels

JIS symbol(3

Effective diameter(3[mm]

Heat treatment [℃]

JIS hardness [HB]

Design hardness (4 HRC

Main applications

New

Old

S25C

-

Normalizing

870 to 920 Air cooling

116 to 174

-

Cold forged products, light-duty carburized products

S35C

-

Normalizing

840 to 890 Air cooling

149 to 207

-

Products that do not require thermal refining

S45C

< 35

Thermal refining

830 to 880 Water cooling, 550 to 650 Quick cooling

179 to 255

14* to 26

Light-duty thermal refined products, induction hardened products

S55C

< 40

Thermal refining

810 to 860 Water cooling, 550 to 650 Quick cooling

212 to 277

20* to 29

Light-duty thermal refined products

SCr415

SCr21

< 30

Quenching

850 to 900 Oil cooling, 150 to 200 Air cooling

217 to 302

-

Small carburized products

SCr420

SCr22

< 35

Quenching

850 to 900 Oil cooling, 150 to 200 Air cooling

235 to 321

-

General carburized products

SCr440

SCr4

< 45

Thermal refining

830 to 880 Oil cooling, 520 to 620 Quick cooling

269 to 331

28 to 36

General thermal refined products

SCM420

SCM22

< 45

Quenching

850 to 900 Oil cooling, 150 to 200 Air cooling

262 to 352

-

Heavy-duty carburized products

SCM435

SCM3

< 60

Thermal refining

830 to 880 Oil cooling, 530 to 630 Quick cooling

269 to 331

28 to 36

Heavy-duty induction hardened products

SCM440

SCM4

< 65

Thermal refining

830 to 880 Oil cooling, 530 to 630 Quick cooling

285 to 352

30 to 38

Heavy-duty thermal refined products

SCM445

SCM5

< 70

Thermal refining

830 to 880 Oil cooling, 530 to 630 Quick cooling

302 to 363

32 to 38*

Heavy-duty thermal refined products

SNCM420

SNCM23

< 50

Quenching

Primary 850 to 900 Oil cooling, 150 to 200 Air cooling

293 to 375

-

Carburized products that require high toughness

Secondary 770 to 820 Oil cooling, 150 to 200 Air cooling

Note 1)

In addition to the types listed above, the following structural steels may be used.

Manganese steel (Mangan is increased so that S38C can be oil-quenched. Example: SMn438)

Boron steel (Boron is added to SCr440 to achieve the hardenability equivalent to that of SCM440. Example: SCrB440)

Leaded free-cutting steel (Lead is added to SCM420 to provide a free-cutting property. However, it is inappropriate for heavy-duty gears that require Hertzian stress of over 200 kgf/mm2. Example: SCM420L.)

Note 2)

In the case of alloy steels for automobiles, H steels with specified hardenability bands are used. Example: SCr415H)

Note 3)

The maximum diameter to ensure the mechanical properties specified in JIS. This limit should not be exceeded.

Note 4)

Hardness specified in drawings may not include the upper and/or lower limits of JIS hardness, as in the items marked with an *.